Fluid control devices include various categories of equipment including control valves and regulators. Such control devices are adapted to be coupled within a fluid process control system such as chemical treatment systems, natural gas delivery systems, etc., for controlling the flow of a fluid therethrough. Each control device defines a fluid flow-path and includes a control member for adjusting a dimension of the flow-path. For example, FIG. 1 depicts a known regulator assembly 10 including a valve body 12 and an actuator 14. The valve body 12 defines a flow-path 16 and includes a throat 18. In FIG. 1, the regulator assembly 10 is configured in a flow-up configuration. The actuator 14 includes an upper actuator casing 20, a lower actuator casing 22, a diaphragm subassembly 30 including a diaphragm 32, and a control member 24.
The control member 24 is disposed within the upper and lower actuator casings 20, 22 and is adapted for bi-directional displacement in response to changes in pressure across the diaphragm subassembly 30. So configured, the control member 24 controls the flow of fluid through the throat 18. As illustrated and in most applications, the surface proximate the lower end of the control member 24 has a generally convex surface past which the fluid flows when the regulator assembly 10 is in the open position. Additionally, as is depicted, the regulator assembly 10 includes a seat ring 26 disposed in the throat 18 of the valve body 12. When the outlet pressure of the valve body 12 is high, a sealing surface 28 of the control member 24 may sealingly engage the seat ring 26 and close the throat 18. Similarly, absent any pressure in the actuator 14 or upon the failure of the diaphragm 32, a coil spring 34 disposed within an annular cavity portion 36 of the upper actuator casing 20 biases the control member 24 into the closed position. Such a regulator is commonly known as a “fail close” regulator.
“Fail open” regulators operate similar to “fail closed” regulators; however, upon failure of the diaphragm, a spring biases the control member open, rather than closed. Examples of “fail open” regulators are illustrated and described in U.S. Pat. Publ. No. 2008/0078460 A1 by Roper et al., entitled “Positioning Device for Pressure Regulator,” which is expressly incorporated by reference herein in its entirety. In regulators such as those taught by Roper et al., the spring may be provided within the control member 24 or otherwise coupled thereto to bias the control member 24 toward the open position. When the diaphragm or other control component fails, fluid continues to flow through the regulator uninterrupted and uncontrolled because the spring opens the regulator assembly. Such configurations therefore often include a monitor regulator, which controls the fluid flow when the “fail open” regulator fails.
In “fail open” regulators such as those taught by Roper et al., it has been observed that operational issue may arise where the “fail open” regulators are installed in high pressure situations. High inlet pressure accompanied by low output pressure can cause control and stability problems for regulators due to additional erratic forces acting on the valve plug. In some instances, these forces can be minimized by increasing the downstream volume of the fluid (i.e., increasing the diameter of the downstream piping) and/or restricting the flow to and from the actuator diaphragm chambers. However, the control problem can sometimes reoccur at higher flow rates even where these corrective measures are implemented, where the force gradient acting on the valve plug causes control issues. In such high flow rate applications, a negative pressure gradient can occur wherein a pressure drop across the valve seat can cause the valve plug to be initially pulled toward the valve seat until spring force overcomes the force produced by the negative pressure gradient, the valve plug my settle into a pattern of high frequency oscillations as the actuator operates to control the response of the regulator. This unstable output may sustain itself due to a lack of stiffness in the actuator system. Therefore, a need exists for an improved “fail open” regulator that maintains a stable output in installations have high inlet pressures, low output pressures, and high flow rates.